1. Field of the Invention
The present invention relates generally to fluid reaction surfaces, and more specifically to a turbine airfoil with a squealer tip.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
A gas turbine engine uses a compressor that produces a compressed air fed into a combustor and burned with a fuel to produce a hit gas flow. This hot gas flow is passed through a turbine which progressively reduces the temperature of the hot gas flow and converts the energy into mechanical work by driving the turbine shaft. Designers are continuously looking for ways to improve the engine performance. Raising the temperature of the hot gas flow will increase the efficiency of the engine. However, the temperature is limited to the material properties of the first stage vane and blade assembly. Designers have come up with complex cooling passages for cooling these critical parts in order to allow for the hot gas flow temperature to exceed the melting temperatures of these parts.
Another way to improve the performance of the engine is to reduce the leakage flow between the rotor blade tip and the outer shroud that forms a seal with the tip. Because the engine cycles through temperatures, the tip clearance varies. Sometimes, the tip touches against the shroud, causing rubbing to occur. Rubbing can damage the blade tips. Providing a larger tip clearance will reduce the chance of rubbing, but will also allow for more hot gas flow to leak across the gap and expose the blade cap to extreme high temperature. Cooling of the blade tip is required to limit thermal damage. Separate blade tip cooling passages have been proposed.
High temperature turbine blade tip section heat load is a function of the blade tip leakage flow. A high leakage flow will induce high heat load onto the blade tip section, and therefore blade tip section sealing and cooling have to be addressed as a single problem. The prior art have proposed a turbine blade tip to include a squealer tip rail which extends around the perimeter of the airfoil flush with the airfoil wall to form an inner squealer pocket. The main purpose of incorporating a squealer tip in a blade design is to reduce the blade tip leakage and also to provide the rubbing capability for the blade. FIG. 1 shows a typical prior art squealer tip cooling arrangement. The blade has a pressure side 12, a suction side 13, and a top 14 with a tip rail 15 extending along the top edge from the trailing edge around the leading edge before stopping short of the trailing edge on the pressure side 12. Film cooling holes 17 are built-in along the airfoil pressure side tip section from the leading edge to the trailing edge and provide edge cooling for the blade pressure side squealer tip. In addition, convective cooling holes 16 also built-in along the tip rail 15 at the inner portion of the squealer pocket 14 provide for additional cooling for the squealer tip rail 15. Secondary hot gas flow migration 21 around the blade tip section is also shown in FIG. 1. The secondary leakage flow 21 over the tip turns into a vortex flow 22 on the blade suction side 13.
FIG. 2 shows an enlarged view for the blade trailing edge squealer tip section. Since the blade tip rail 31 is cut-off at the aft section of the pressure side, it becomes a single squealer tip rail configuration and thus decreases the ability to reduce the blade tip leakage flow. Meanwhile, the suction side blade tip rail 32 is subjected to heating from three exposed sides. Cooling of the suction side squealer tip rail 32 by means of a row of discharge film cooling holes 34 along the blade pressure side peripheral and cooling holes 35 at the bottom of the squealer floor 36 becomes insufficient. Trailing edge cooling slots 33 provide trailing edge cooling. This is primary due to the combination of squealer pocket geometry and the interaction of hot gas secondary flow mixing The effectiveness induced by the pressure film cooling and tip section convective cooling holes is very limited.
It is therefore an object of the present invention to provide for cooling and sealing of an airfoil squealer tip along the trailing edge region of the blade.